1. Field of the invention
The invention is in the field of integrated optical components. The invention comprises an integrated optical polarization splitter based on the mode filter principle.
2. Background to the invention
Optical polarization splitters, also termed TE-TM mode splitters, are used, for example, in coherent optical detection systems. Such systems solve the problem that the polarization state of the light carrying information, which light is usually presented via a standard monomode glass fiber, may fluctuate, inter alia due to the use of the so-called polarization diversity (PD) system. According to this system, the light received is split into two components having mutually perpendicular polarization directions with the aid of a polarization splitter. The two components are then detected and processed separately. An integrated coherent optical detection system requires, of course, an optical polarization splitter in integrated form. Hitherto known polarization splitters have been based, inter alia, on the principle of mode filtering. This principle implies that, in the case of waveguides which approach one another, the optical field distribution in the interaction region where the waveguides come together depends on the extent to which the waveguides resemble one another.
If they are identical, two independent waves are produced in said interaction region: a so-called even mode and a so-called odd mode of equal intensity. If the guides are, however, different, i.e. asymmetrical, for example in the width, the even and the odd modes are no longer equally intensely initiated: if the light enters via the guide with the lowest propagation constant, the odd mode becomes more intense, but via the other guide it is precisely the even mode which is initiated more intensely. If the asymmetry between the guides is just great enough, only one mode is initiated. However, it is the case that the greater the angle between the guides, the greater the asymmetry required. Precisely the opposite applies for waveguides which remove the light from the interaction region and therefore recede from one another: if the asymmetry is sufficient, the even mode proceeds completely in the outgoing guide having the highest propagation constant and the odd mode precisely via the other.
Thus, an asymmetry provided between the incoming waveguides produces a selective initiation of the even or odd mode in the interaction region; an asymmetry in the outgoing guides produces a selective coupling out of the even or odd mode in each of the outgoing waveguides. The direction of the asymmetry determines which mode belongs to which guide. When applied to a polarization splitter, this means that the asymmetry is so chosen that it becomes different in sign for the TE polarization than for the TM polarization at right angles thereto. This may occur either at the input side or at the output side. If the output side is equipped in this manner and the input side is so constructed that it comprises an input channel which can conduct both polarizations in the fundamental mode, this polarization splitter operates as follows. The incoming waveguide injects light which contains both the TE and the TM polarization. Each of these polarizations in said (even) fundamental mode will then be coupled out at the output side via the guide having the highest propagation constant for that polarization. In this way, splitting of polarizations is brought about.
Such a polarization splitter based on a mode filter is known from reference [1] (see C). This known splitter is provided on a substrate of LiNbO.sub.3 (zero-degree cut) and is based on a combination of a standard optical waveguide obtained by Ti diffusion and a polarization-sensitive optical waveguiding side branch connected thereto at an acute angle (.theta.). The polarization-sensitive optical waveguide is provided in/on the substrate by means of a "proton-exchange" (PE) process. This process achieves the result that the extraordinary refractive index n.sub.e has increased at the position of the guide, while the ordinary refractive index n.sub.0 has decreased somewhat. The Ti waveguide and the PE side branch can have mutually different widths and the connection of the PE branch is tapered.
However, this known polarization splitter requires a very critical geometry and is therefore fairly laborious from a production-engineering point of view. A good splitting of the polarizations moreover requires a very acute angle (.theta..ltoreq.0.01 rad for a suppression of up to 20.0 dB), with the result that the required length is relatively great.